Secondary recovery technique



March 8, 1960 A` DRAPER 2,927,637

SECONDARY RECOVERY TECHNIQUE Filed Sept. 13, 1956 RESERVOIR@ WATER OIL INJECTED WATER OIL INJECTED WATER OIL INJECTED WATER FLOOD Arthur L. Draper Inventor Bylw07 Wmtorney v oil-bearing subterranean formations.

Verations.

. crete banks.

Unite SECONDARY RECOVERY TECHNIQUE Arthur L. Draper, Tulsa, kla., assignor, by mesne assignments, to Jersey Production Research Company The present invention is broadly concerned with an improved method for the recovery of crude petroleum from The invention is more specifically concerned with an improved technique utilizing a particular combination of water banks and oil banks as a driving medium. In accordance with the present invention, a secondary recovery operation is carried out wherein the formation is flooded employing a plurality of alternate Water and oil banks.

It is well known -in the art to use various methods and techniques for removing oil in secondary recovery op- For example, it has -been suggested that gas be linjected and that the reservoir be repressured. It is also known in the art to `inject Water at an injection Well and to cause this water to flow toward a producing well driving oil ahead of it to the producing well. Another procedure suggested-has been to introduce a bank of low-boiling hydrocarbon such `as propane' and to drive this bank by means of either water or natural gas toward the producing well. These -methods have been successful to some degree. However, in many instances, the beneficial results have not been significant.

In accordance with the present invention 4an alternate injection technique is employed wherein a plurality of alternate banks of Water and oil are passed or caused to flow from an injection well or wells to a producing well or wells. In accordance with a preferred adaptation of the invention the injected oil is of a higher viscosity than `the oil being recovered from the reservoir when the reservoir is characterized by water wettability. When the reservoir is characterized by oil wettability, the preferred adaptation of the invention would `be the use of an injected oil of a lower viscosity than the oil being recovered.

yit is critical in the practice of the present invention that the lalternate bank injection flooding technique be carried out so that the individual banks remain discrete and separate. In particular, the oil banks should not bypass the water banks. If they are permitted to bypass or outrun the water banks, the oil banks mix with the oil in the formation and are produced ahead of the water banks with no appreciable increase in oil recovery. rlhe process then merely behaves like a waterllood.

To obtain a good volumetric iiushing efficiency in every part of a reservoir in accordance with this invention, a volume being iiushed in any part of the reservoir should have discrete and separate alternate banks of water and oil passing through it. In this manner, an increase in oil recovery Ais obtained with the oil injected in the banksr produced from the formation still as dis- The injected oil is entirely recovered from the reservoir together with an amount of oil greater than would have been recovered by straight waterooding.

As stated earlier herein, it is considered that the application of the process of this invention to field reservoirs the use of an oil more viscous than the oil in place should be used when the reservoirs are water-Wet. The use of the more viscous oil in these instances results in an additional improved oil recovery apparently because States arent fie of the tendency of the more viscous oil to overcome the inefficiencies inherent in straight waterooding only imposed by normal stratification occuring in reservoirs. The injection of a material more viscous than the oil in place as a llooding material results in a more uniform displaceent from the reservoir. Also, because of the higher viscosity o-f the oil used in the alternate driving banks of `water and oil, a more favorable sweep of the reservoir is obtained-ie., more of the reservoir is contacted by the injected liuids passing from an injection well to a production well. Therefore, the reduction of the eifects of reservoir stratification and the increase in thev portion of the reservoir swept coupled with the improved liushing efficiency of every reservoir volume contacted by the alternate and discrete banks of water and oil yields a substantial improved recovery of the oil in place.

While the oil recovery which may be attributed to decreasing the effects of stratication and increasing the swept portion is lof practical importance, the critical eiect is created by the alternate water and oil banks moving through the reservoir. These banks result ina lower residual oil and therefore in a higher oil recovery than other various methods and techniques for removing oil from the reseivoir in secondary recovery operations. A method and technique for insuring the continuous separation of the water `and loil banks as they pass through the reservoir is of basic importance to the practice of the invention as the discrete banks or slugs of water and oil are driven by the subsequent water injection to yield the improved oil recovery.

In accordance with the present invention, the mobility or mobility factor of the oil banks or slugs should be about equivalent to the mobility or mobility factor of the injected water bank or slug. If this be done, the tendency for the respective injected alternate banks to mix and outow or Outrun one another is substantially eliminated. The mobility factor of water, for example, may be expressed as wherein The mobility factors compared for the purposes of this invention are those of water at residual oil saturation and of oil at residual Water saturation.

Thus in accordance with the present invention, if kw equals l0() millidarcies (md.) and Mw equals 0.80 centipoise (cp), then the mobility of water equals:

ku. M-w equals m equals rnd/cp.

if the formation relative permeability of oil equals 200 niillidarcies (md), -then M a M, equals 1.6 ep.

It is thus essential that under the conditions specified, the viscosity of the injected oil Mo equals 1.6 c

125 equals equals 62.5

Mo'equals 3.2 cp.

From the above it is apparent that the driving injected oil must have a viscosity which is a function of the viscosity of the water and the relative permeability factors of the injected oil and water so that the alternating banks of injected oil and water will have the same mobility factor.

The process of the present invention may be more fully understood by reference to the drawing illustrating one embodiment of the same. Referring speciiically to the drawing, a producing formation 1 is shown positioned below the earths surface 2. Bore holes 3 and i extend from the earths surface to the producing formation. in accordance with the concept of the present invention, alternate banks of water and oil are injected into formation 1 through bore hole 3 by means of suitable pumping equipment and the like. The condition of the reservoir is 62.5 equals illustrated in the drawing just prior to the point when the iirst injected bank of water 5 reaches production bore hole 4. At thisk condition, reservoir oil 6 is being driven yahead of waterbank 5 and being produced from production bore hole 4. The water bank 5 in turn is being driven by oil bank 8 which in turnv is being driven by water bank 9 which in turn is being driven by oil bank 10. At this point the bore hole 3 is lled with water which is driving the respective alternate banks. in the particular operation described, three banks of injected oil are injected and thus the entire reservoir, after oil bank 12 reaches bore hole 4, will be filled with Water. While the operation has been described utilizing three injected banks of oil, it is to be understood that as many as six slugs or banks may be employed or even more depending upon the nature of the reservoir conditions. However, in general, itis preferred to use from two to four injected slugs or banks of oil alternately with banks of water. The oil injected in the banks or slugs is recovered from the reservoir as oil production so that the choice of the optimum number of banks of oil to be used in a particular application may be determined for that reservoir by laboratory measurements. The optimum number is set by the maximum additional oil recovered by the least number of banks whose injection and subsequent passage through the reservoir by the following water drive therefore take the least time. This procedure corresponds to the most economically attractive method of employing the process.

The process of the present invention may be further understood by the following illustrative examples which obviously however `are not intended to be limiting with respect to iield procedure or operating conditions. The two examples show two ways of preserving the bank separation. The iirst employs larger water banks with oil banks of oil having the same viscosity as the oil in place. The second employs oil banks of oil having a higher Viscosity than the oil in place but with the same mobility as the water in the water banks. Both operations are equivalent in the recovery of additional oil but the second method is considered superior for field operations.

t Examples of Q rings to anges at the end to permit ooding of the core. The core material was taken from an outcrop of Torpedo sandstone.

A straight waterood of the test core Was made to give a reference behavior for the process. Two experiments were made on the alternate injection technique. After every experiment, the core was restored to its equivalent initial oil and connate water saturations by completely saturating the core with brine. The initial oil saturation was then established by ushing the core with a high viscosity (approximately cp.) oil which was subsequently replaced by a low viscosity (1.3 cp.) oil typical of many original formation oils suited to waterilooding. The formation relative permeability to water at residual oil saturation (kw) was approximately millidarcies. The viscosity of the salt water or brine used in the formation had a viscosity (Mw) at room temperature of approximately 1.0 centipoise. The mobility is given by 125 millidarcies/centipoise the oil viscosity (M0) is therefore equal to 4.8 centipoises.

Two separate flooding experiments were made under the above conditions. In one, the oii used in the injected oil banks was the same oil as the oil in place and therefore had the same viscosity. The water banks used to separate the oil banks were five times as large as the oil banks. This experiment is designated Experiment 1. In the other experiment, the oil used in the injected oil banks was a dilferent oil from the oil in place in the test core and had a viscosity of 5.0 centipoises. The oil banks were injected alternately with water banks which were `of the same equivalent size. This experiment is designated Experiment 2. ln both experiments the banks or slugs were driven through the core by a water flood.

The results of the experiments are as follows:

Experiment 1 Experiment 2 Pore Volume (PV) of Corn, cc Initial Oil Saturation, cc Initial H2O Saturation, cc Viscosity of Oil in Place, cps. Viscosity of Injected Oil, cps..-

Volume of Water Banks, cc. Number of Water Banks used Volume of Oil Banks, cc Number of Oil Banks Used Total Oil Volume Injected. cc Total Oil Volume Produced, cc. Injected Volume of Oil, cc Net Volume of Original Oil in Place Produced. cc Comparisons: Comparisons of oil recovery by the reference Water flood and the alternate banks method are as follows:

Volume of Original Oil in Place Produced in Reference Watertlood, cc

Increase in Volume of Oil Rccovcrcd by Method of Invention, cc

Percentage Increase in Oil Rccovery 152 (1.0 PV) 152 (1.0 PV) 122 122 12 (0.079 Pv?) 12 (0.079 PVS a s0 (0.237 Pv) 112 75 respective volumes of injected oil banks be substantially This sandstone material is char-v acterized as being hydrophilic or preferably water wet. Y

equivalent to one another and that the volume of the injected water banks be substantially equivalent to each other. It is preferred that the respective volumes of the oil banks to the volumes of the water banks are such that the oil banks not exceed the volumes of the water banks. It is preferred that the volume of the oil banks be from about 25% to 75% of the volume of the water bank.

The volumes of the individual banks or slugs of oil utilized are in the range of from about 0.5 to of the pore volume of the reservoir being processed. A preferred range for the individual oil bank is from about 2% to 4% of the pore volume ofthe reservoir.

What is claimed is:

1. A process for the recovery of oil from a preferentially water-wet subterranean reservoir which comprises passing a plurality of alternate banks of water and oil from an injection well through the reservoir to a production well, preselecting the injected oil to have a mobility about equivalent to the mobility of the injected water, further preselecting the injected oil to have a viscosity least equal to the viscosity of the reservoir oil.

2. A process as dened in claim 1 in which the injected oil has a viscosity greater than that of the reservoir oil.

3. A process as defined in claim l in which the banks of water are substantially equal in volume to one another and the oil banks are also substantially equal in volume to one another.

4. A process as dened in cl-am 3 in which the volume of each oil bank is about 25 to 75% of the volume of each water bank.

5. A process for the recovery of oil from a preferentlally water-wet subterranean oil reservoir which comprise injecting a plurality of alternate banks of water and oil through the reservoir from an injection well to a production well, preseleeting the injected oil to have a mobility about equal to that of the injected water and a viscocity greater than that of the reservoir oil, the banks of water being equal in volume to one another and the banks of oil also being equal in volume to one another, each bank of oil having a volume between about 25% and 75% of the volume of each water bank and constituting about 0.5% to 10% of the pore volume ofthe reservoir being processed.

6. A process as dened in claim 5 in which each oil bank has a volume constituting about 2% to 4% of the pore volume of the reservoir being processed.

References Cited in the die of this patent UNITED STATES PATENTS 2,669,306 Teter et al Feb. 16, 1954 

